1. Industrial Field of the Invention
The present invention relates to a process for forming a silicon oxide film (SiO.sub.2 film) on a semiconductor substrate.
2. Prior Art
In the fabrication of a MOS (metal oxide semiconductor) semiconductor device, it is essential to form a gate oxide film by oxidation. The properties of a gate oxide film are greatly influenced by the atmosphere under which the film is formed. Accordingly, an oxide film can be deposited by processes differing from each other in atmospheric conditions. For instance, a silicon oxide film can be deposited by any of the processes belonging to, for example, dry oxidation and wet oxidation. The dry oxidation process comprises supplying a sufficiently dried high purity oxygen to the surface of a hot silicon substrate. In this manner, a silicon oxide film can be formed on the surface the silicon substrate. The wet oxidation process comprises supplying, to the surface of a silicon substrate, a hot carrier gas containing water vapor. Thus, this process also provides a silicon oxide film on the surface of the silicon substrate.
A pyrogenic oxidation is one of the wet oxidation processes. This method is characterized in that pure hydrogen is combusted to generate water. Accordingly, a highly reproducible oxidation can be effected without controlling water flow. With respect to hot carrier resistance of the silicon film, a film obtained by this method is inferior to a one obtained by a dry oxidation process. This is because a film obtained by pyrogenic oxidation contains a large amount of electron traps attributed to water. However, the pyrogenic oxidation process is far superior to the dry oxidation process considering the insulator breakdown voltage and the long-term reliability of the silicon oxide film. It is also reported that pyrogenic oxidation is employed in the fabrication of flash memories, i.e., novel types of memory whose production is expected to increase acceleratedly.
On the other hand, it is known that a dry oxidation method comprising forming a silicon oxide film under an oxidizing atmosphere based on a compound containing chlorine (e.g., HCl, Cl.sub.2, CCl.sub.4, C.sub.2 HCl.sub.3, CH.sub.2 Cl.sub.2, and C.sub.2 H.sub.3 Cl.sub.3) or a compound containing a halogen atom other than chlorine provides a silicon oxide film which yields the following advantageous effects (this method is hereinafter referred to as "hydrochloric acid oxidation method"):
(A) neutralization or gettering of alkali metal impurities incorporated in the silicon oxide film; PA1 (B) reduction of stacking defects; PA1 (C) improvement in time-zero dielectric breakdown (hereinafter abbreviated and referred to as "TZDB"), i.e., an index for evaluating the resistance against dielectric breakdown in a short period of time; and PA1 (D) improvement of channel mobility. PA1 (E) reduction of fixed charge; and PA1 (F) reduction of the density of states (interface levels). PA1 heat treating a semiconductor substrate at a temperature of 1,150.degree. C. or higher under an atmosphere of 100% hydrogen gas; and PA1 forming a silicon oxide film on the semiconductor substrate by wet oxidation, followed by heat treating the silicon oxide film under an inert gas atmosphere containing a halogen element.
It is also well-established that, by heating the silicon oxide film once formed on a silicon substrate in the temperature range of from 800.degree. to 1,000.degree. C. under an atmosphere of an inert gas such as nitrogen and argon for a duration of about 30 minutes, the following effects attributed to the favorable interface between the silicon oxide film and the silicon substrate can be obtained:
Time dependent dielectric breakdown (hereinafter abbreviated and referred to as "TDDB") provides an index for evaluating the long-term reliability of a silicon oxide film. The TDDB does not occur immediately at the instant an electric current stress or a voltage stress is applied; the breakdown occurs on the silicon oxide film only after time passes for a certain period from the point the stress is applied.
A silicon oxide film formed by the wet oxidation process yields superior TDDB characteristics as compared with a one obtained by the dry oxidation process. This signifies that the wet oxidation process provides silicon oxide films having superior long-term reliability. It is believed that the reason for this is attributed to the presence of hydroxyl groups (--OH) and silanol groups (--SiOH.sub.x) within the silicon oxide film, which contribute to the improvement of the TDDB characteristics.
The hydrochloric acid oxidation process, which is a type of dry oxidation process, yields silicon oxide films having improved TZDB characteristics. However, no improvement can be observed on the TDDB characteristics of the films obtained by this process. Furthermore, the hydrochloric acid oxidation process requires strict control of the apparatus and the conditions for forming the oxide film.
As described in the foregoing, a silicon oxide film obtained by a wet oxidation process has excellent TDDB characteristics. By further heat-treating the silicon oxide film obtained by this method under an inert gas such as nitrogen and argon, improvements concerning the reduction of fixed charges and of the density of states (refer to the point (E) and (F) enumerated above) can be certainly achieved. However, the TZDB characteristics of the silicon oxide films are still inferior to those obtained by hydrochloric acid oxidation process.
Furthermore, for instance, JP-A-3-219632 (the term "JP-A-" signifies an "unexamined Japanese patent publication", hereinafter) discloses a process for forming an insulator film, which is characterized by thermally treating an already formed insulator film under a reactive gas atmosphere containing, for example, chlorine. According to this process, the insulator film is formed by rapidly heating the substrate using an infrared-emitting lamp. That is, from a broader viewpoint, the insulator film is formed by a dry oxidation process. Accordingly, the TDDB characteristics of the silicon oxide film obtained by this process is still inferior to that of the film formed by a wet oxidation process. An object of the proposed method is to reduce the defects ascribed to dangling bonds and the like which are formed during the formation of the film. It can be seen that the process disclosed in this patent publication is not intended to improve the TZDB or the TDDB characteristics of the oxide film. Furthermore, the disclosed process comprises effecting the heat-treatment by a so-called RTA (rapid thermal annealing) at 1,000.degree. C. for a duration of 20 seconds to form an SiCl.sub.x doping layer in the vicinity of the interface between the insulator film and the substrate.
It can be seen from the foregoing that any of the conventional dry oxidation processes and wet oxidation processes fails to provide a silicon oxide film which suffices both demands on TZDB and TDDB characteristics.
The present applicants proposed, in Japanese patent application Hei-5-86836 (filed on Mar. 23rd, 1993), a process for forming a silicon oxide film having satisfactory characteristics concerning both TZDB and TDDB. The process disclosed in this application is certainly effective; however, some problems, which are explained below, are yet to be solved.
The characteristics of a gate oxide film are influenced not only by the atmospheric conditions under which the film is formed, but also by the nature of the underlying semiconductor substrate. More specifically, minute defects such as stacking defects and oxygen precipitates in the substrate are the factors which deteriorate the quality of the gate oxide film. The problem concerning the minute defects has been coped with by heat treating the substrate at a high temperature, i.e., by forming a denuded zone. The denuded zone is a defect-free layer formed by externally diffusing the oversaturated oxygen. However, with further progress in the present day technology, it has been found that Czochralski(CZ)-grown silicon wafers used in the present-day fabrication of MOS semiconductor devices contain, in addition to various types of minute defects induced during the heat treatment of the process, residual defects which are formed during the crystal growth of the single crystal. Since those defects remain at a density of from 10.sup.5 to 10.sup.6 /cm.sup.3 even after a heat treatment at a relatively high temperature, they cause deterioration of the film and lower the withstand voltage when they are taken into the gate oxide film. Thus, those defects further increases the B-mode failure in the TZDB characteristics.
It is known that the use of a semiconductor substrate fabricated from CZ-grown single crystals pulled up at a low rate or an epitaxially grown semiconductor substrate improves the breakdown withstand voltage characteristics of the silicon oxide thin film formed thereon. However, the fabrication cost of such substrates are high. Moreover, the semiconductor substrates fabricated from a single crystal pulled up at a low rate tend to be influenced by the heat treatment performed during the fabrication of the semiconductor device. Accordingly, the practical effect concerning the use of such a substrate is still questioned.